Method of producing nanostructured metals using high-intensity ultrasonic vibration

ABSTRACT

A method of producing a nanostructured article includes simultaneously subjecting a body of material to external force and vibration to produce a desired nanostructure in the body of material.

The United States Government has rights in this invention pursuant tocontract no. DE-AC05-00OR22725 between the United States Department ofEnergy and UT-Battelle, LLC.

FIELD OF THE INVENTION

The present invention relates to methods of producing nanostructuredmaterials, and more particularly to methods of producing nanostructuredmaterials, especially metals (including alloys and metal-matrixcomposites), wherein a combination of external force, especiallycompressive force, and vibration, especially ultrasonic vibration, isused to process solid material to produce improved nanostructurestherein.

BACKGROUND OF THE INVENTION

Nanostructured materials offer unique and entirely different mechanical,electrical, optical, and magnetic properties compared with conventionalmicrostructured or millimeter-scaled materials. For example, thehardness of nanocrystalline copper is known to increase with decreasinggrain size; nanostructured copper having 6 nm grains can have as much asfive times the hardness of conventionally prepared copper. Anotherexample is nanostructured Al—Ni—In alloys, which are known to exhibit atensile strength (σ_(f)>1200 MPa) greater than conventionalhigh-strength aluminum alloys. Nanostructured M50 steel is more fatigueand fracture resistant than conventional M50 steel that is widely usedin the aircraft industry as the main-shaft bearings in gas turbineengines.

Conventional methods for producing nanostructured materials include gasatomization, ball milling followed by consolidation, and rapidsolidification. Such processes tend to be expensive and prone tocontamination. Recent approaches for producing nanostructured materialsinclude severe plastic deformation. Equal Channel Angular Extrusion(ECAE) is one of the methods that use severe plastic deformation toproduce nanostructured materials but it is an expensive method forproducing nanostructured materials.

OBJECTS OF THE INVENTION

Accordingly, objects of the present invention include the provision ofmethods of processing metal bodies to produce desired nanostructurestherein. Further and other objects of the present invention will becomeapparent from the description contained herein.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, the foregoingand other objects are achieved by a method of producing a nanostructuredarticle that includes simultaneously subjecting a body of material toexternal force and vibration to produce a desired nanostructure in thebody of material.

In accordance with another aspect of the present invention, a method ofproducing a nanostructured metal article that includes simultaneouslysubjecting a metal body to external compressive force and ultrasonicvibration so that a desired nanostructure is produced in the metal body.

In accordance with a further aspect of the present invention, apparatusfor processing a body of material includes means for applying externalforce to a body of material in combination with a vibrator disposed forsimultaneously applying vibration to the body of material to produce adesired nanostructure in the body of material.

In accordance with yet another aspect of the present invention,apparatus for processing a metal body includes means for applyingexternal compressive force to a metal body in combination with anultrasonic vibrator disposed for simultaneously applying ultrasonicvibration to the metal body to produce a desired nanostructure in themetal body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a test arrangement fortesting the present invention.

FIG. 2 is an approximately 3× magnified photograph showing the tip of acone specimen that was deformed in accordance with the presentinvention.

FIG. 3 a is a scanning electron microscopy (SEM) image of themicrostructure at the deformed tip shown in FIG. 2.

FIG. 3 b is a transmission electron microscopy (TEM) image of themicrostructure at the deformed tip shown in FIG. 2.

FIG. 4 a illustrates an embodiment of a continuous method of carryingout the present invention.

Like elements in the figs. are called out with like numerals.

FIG. 4 b illustrates another embodiment of a continuous method ofcarrying out the present invention.

FIG. 5 a illustrates an embodiment of a continuous method of carryingout the present invention using a roll feed.

FIG. 5 b illustrates another embodiment of a continuous method ofcarrying out the present invention using a roll feed.

FIG. 6 a is a graph representing applied forces in an ultrasonicprocessing method.

FIG. 6 b is a graph representing applied forces in a combined ultrasonicand compression processing method in accordance with the presentinvention.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above-described drawings.

DETAILED DESCRIPTION OF THE INVENTION

Solid materials subjected to vibration, especially high intensityultrasonic vibration, undergo alternating tensile and compressivestresses and/or strains. Under the influence of such alternating forces,beneficial vacancies and dislocations are induced but the material issubject to fatigue failure due the tensile forces. The basic concept ofthe present invention is to simultaneously use external force (forceapplied to the exterior of a work-piece), preferably externalcompressive force, to the work-piece (a metal body, for example) subjectto vibration. Because of the composite nature of the forces/stresses,the alternating tensile/compressive stresses are modified intoalternating compressive forces, reducing pernicious tensile forces andpreventing materials from undergoing fatigue failure. Shear forces andeven some tensile forces may remain and even may be beneficial to theinvention.

External force can be applied by any means, such as, for example,compressive force, magnetic force, and combinations of the foregoing.External compressive force can be applied to a metal body by any of thevarious and sundry known methods of metalworking such as, for example,extrusion, swaging, hammering, pressure, forging, etc.

Vibration, preferably ultrasonic vibration, can be applied to the metalbody by any vibrator, preferably an ultrasonic vibrator, capable ofproducing sufficiently intense vibration, and can be applied directly tothe work-piece or indirectly, such as through the body of an extrusiondie, magnet, anvil, or press ram.

FIG. 1 shows a schematic illustration of a test of the method. Aspecimen work-piece having a conical tip with a length of an integralmultiple of a half wave-length of the ultrasonic wave is connected to anultrasonic horn and an ultrasonic generator. High-intensity ultrasonicenergy is then injected into the tip of the cone. Alternatingcompressive stresses are thus generated at the tip of the cone specimenpartly due to ultrasonically induced stresses and strains and partly dueto the weight of the ultrasonic horn/generator assembly. As a result,severe plastic deformation occurs at the tip of the specimen.

EXAMPLE I

A metal cone specimen was subject to ultrasonic energy as describedabove. FIG. 2 shows the tip of the deformed cone specimen. The sharp tipof the cone becomes umbrella-shaped. FIG. 3 a is a SEM image of themicrostructure at the deformed tip and FIG. 3 b is a TEM image of thegrains in the deformed region. The grain sizes in the deformed tip areabout 100 nm.

The method described above can be adapted and modified into a continuousprocess for the production of wires having nanostructured grains. FIGS.4 a and 4 b illustrate some embodiments of continuous methods ofcarrying out the present invention to form wires using ultrasonicvibration and external forces to cause severe plastic deformation of thework-piece, and use dies to collect the deformed material. Furtherinformation relating to severe plastic deformation methods can be foundin U.S. Pat. No. 6,895,975 issued on May 24, 2005 to Chaudhury, et al.entitled “Continuous Severe Plastic Deformation Process for MetallicMaterials”, the entire disclosure of which is incorporated herein byreference. Some embodiments of the invention use a die similar to thetype of die used in equal-channel-angular-extrusion (ECAE) processes.

FIG. 4 a shows a die 20 having a die channel 22 with a sharp corner 24for causing severe plastic deformation of the work-piece (metal body,for example) 28, usually a wire. An ultrasonic vibrator 14 is shown incontact with the feeding end of the work-piece 28. Ultrasonic vibrationis injected into the work-piece 28 as it is pushed through the die 20 toproduce bulk nanostructured wire.

FIG. 4 b shows a die 30 having a die channel 32 with a sharp corner 34for causing severe plastic deformation of the work-piece 28, usually awire. An ultrasonic vibrator 14 is shown in the die and in contact withthe work-piece 28 as it passes through the die channel 32. Ultrasonicvibration is injected into the work-piece 28 as it is forced through thedie 30 to produce bulk nanostructured wire.

Nanostructured wire produced by the present invention is free fromcontaminants such as oxidation and surface contamination that usuallyoccurs that use ball milling and rapid solidification. Moreover,nanostructured wire produced by the present invention is free fromporosity formation that occurs in methods that use condensation of smallparticles or droplets.

In accordance with the present invention, vibration at an ultrasonicfrequency is operably applied at a frequency in the range of 1 Hz to 150MHz, preferably in the range of 10 kHz to 25 kHz, and at a powerintensity greater than 200 W, preferably in the range of 500 W to 2000W. The duration of ultrasonic processing can be anywhere in the range of0.1 second to 20 minutes. Once the beneficial results of ultrasonicprocessing are achieved, continued subjection of the process material isnot deleterious, therefore duration is not considered to be a criticalparameter.

The amount of the external force should be larger enough to modify thealternating tensile/compressive stresses (forces) induced by thehigh-intensity ultrasonic vibration into mainly alternating compressiveand shear stresses (forces). It is necessary to prevent materials fromundergoing fatigue failure under high-intensity ultrasonic vibrations.Generally the external force can be high but not too high to causedimensional instability or even the failure of the materials to beprocessed.

Referring to FIG. 6 a, a sine wave 60 represents alternating tensile andcompressive forces caused by ultrasonic vibration. Line 62 representszero force, arrow 64 represents tensile force caused by ultrasonicvibration, and arrow 66 represents compressive force caused byultrasonic vibration. In FIG. 6 b, external compressive force 68 isapplied, so that sine wave 60′ is offset below the zero force line 62and now represents increasing and decreasing (alternating) compressiveforces and no tensile forces.

The bulk grain size obtained by this invention is about 100 nm bypassing through the material over the ultrasonic radiator. Using adevice similar to ECAE, the material can be processed a few times withfurther grain size reduction after each pass.

The device shown in FIG. 5 a can be used to assist the ECAE process formaterial of large cross-section. The die 30 and ultrasonic vibrator 14are similar to that shown in FIG. 4 b. Rollers 52 are used to force themetal work-piece 28 through the die channel 32 with sharp corner 34.

FIG. 5 b shows another embodiment of the invention wherein ultrasonicvibration is applied to the die. The ultrasonic vibrator 14 appliesultrasonic vibration to the die body 50. Rollers 52 are used to forcethe metal work-piece 28 through the die channel 22 with sharp corner 24.

The application of high-intensity ultrasonic vibration brings about twoeffects: One is the acoustic “softening” of materials (because thedislocations are dislodged and moved by the ultrasonically inducedinstantaneous stresses/strains) and the other is the reduction offriction forces at the metal/die interface.

Due to the first effect, the metal to be extruded becomes soft so itwill be easier to be extruded using the EACE process. This is alsoextremely important for materials that are not ductile or that aredifficult to be extruded using ECAE process. These materials include Mgmetal and alloys, titanium metal and alloys, and other materials withhcp crystal structure.

Due to the second effect, the forces required to push material throughan ECAE die will be greatly reduced. This is also important since it isthe friction force that limited the application of the ECAE process.This is especially true for the extrusion of metal of largecross-section, in which the friction force is so high that basicallythere are no materials tough enough to be used as the die material. Thelargest aluminum 6061 bar that has been extruded using the ECAE processis only a few square inches in cross-section.

The two effects described above can be utilized to assist the ECAEprocess. One embodiment of this invention is to use ultrasonic vibrationand transmit the vibration to the interface of the extruded material andthe ECAE die (for reducing friction force) and to the extruded materialaround the sharp corner of the ECAE die (for softening the material).

FIGS. 5 a, b shows schematically how ultrasonic vibration can be used toassist the ECAE process. Ultrasonic vibration is applied by theultrasonic vibrator 14 to the work-piece of extruded material 28 at thecorner of the ECAE die, where the shear stress and friction stresses arethe largest. Rolls 52 are used to continuously feed the extrudedmaterial 28 through the ECAE die 30. The use of ultrasonic vibrationwill generally soften the material 28 at the corner 34 of the die 30 andreduce the friction between the extruded material 28 and the die 30,significantly reducing the amount of applied force necessary to carryout the ECAE process. A significant issue involved in this embodiment ofthe invention is that the rolls 52 should preferably be positioned atthe antinodes where the ultrasonic vibration is at a minimum. Suchplacement of the rolls isolates the roll feed system from vibration fromthe extruded material.

As can be seen in the description above, the ultrasonic vibrator can bedisposed in contact with the means for applying compressive force, andcan even be supported thereby. Such disposition, although generallypreferable is not, however absolutely necessary. It is critical to theinvention that the relative disposition of the ultrasonic vibrator andmeans for applying compressive force be such that the forces generatedthereby have a combined effect on the metal body.

While there has been shown and described what are at present consideredthe preferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications can beprepared therein without departing from the scope of the inventionsdefined by the appended claims.

1. A method of producing a nanostructured article comprisingsimultaneously subjecting a body of material to external force andvibration to produce a desired nanostructure in the body of material. 2.A method of producing a nanostructured article in accordance with claim1 wherein said external force comprises at least one of the groupconsisting of external compressive force, magnetic force, and shearforce.
 3. A method of producing a nanostructured article in accordancewith claim 1 wherein said vibration comprises ultrasonic vibration.
 4. Amethod of producing a nanostructured article in accordance with claim 1wherein said body of material comprises a metal.
 5. A method ofproducing a nanostructured article in accordance with claim 1 whereinsaid external force and said vibration combine to produce alternatingcompressive stresses in the body of material.
 6. A method of producing ananostructured metal article comprising simultaneously subjecting ametal body to external compressive force and ultrasonic vibration toproduce a desired nanostructure in the metal body.
 7. A method ofproducing a nanostructured metal article in accordance with claim 6wherein said external compressive force and said ultrasonic vibrationcombine to produce alternating compressive stresses in the metal body.8. A method of producing a nanostructured metal article in accordancewith claim 6 wherein said external compressive force comprises a severeplastic deformation process.
 9. A method of producing a nanostructuredmetal article in accordance with claim 6 wherein said severe plasticdeformation process comprises equal-channel-angular-extrusion.
 10. Amethod of producing a nanostructured metal article in accordance withclaim 6 wherein said ultrasonic vibration is applied at a frequency inthe range of 1 Hz to 150 MHz and at a power intensity greater than 200W.
 11. A method of producing a nanostructured metal article inaccordance with claim 10 wherein said ultrasonic vibration is applied ata frequency in the range of 10 kHz to 25 kHz, and at a power intensityin the range of 500 W to 2000 W.
 12. Apparatus for processing a body ofmaterial comprising means for applying external force to a body ofmaterial in combination with a vibrator disposed for simultaneouslyapplying vibration to the body of material to produce a desirednanostructure in the body of material.
 13. Apparatus for processing abody of material in accordance with claim 12 wherein said vibrator isdisposed in contact with said means for applying external force. 14.Apparatus for processing a body of material in accordance with claim 12wherein said vibrator is supported by said means for applying externalforce.
 15. Apparatus for processing a body of material in accordancewith claim 12 wherein said external force comprises at least one of thegroup consisting of external compressive force magnetic force, and shearforce.
 16. Apparatus for processing a body of material in accordancewith claim 12 wherein said vibrator comprises an ultrasonic vibrator.17. Apparatus for processing a metal body comprising means for applyingexternal compressive force to a metal body in combination with anultrasonic vibrator disposed for simultaneously applying ultrasonicvibration to the metal body to produce a desired nanostructure in themetal body.
 18. Apparatus for processing a metal body in accordance withclaim 17 wherein said ultrasonic vibrator is disposed in contact withsaid means for applying compressive force.
 19. Apparatus for processinga metal body in accordance with claim 17 wherein said ultrasonicvibrator is supported by said means for applying compressive force.